The bulk of the electric loads in the modern industrial world requires a precisely regulated AC frequency. The AC electric power in the world is usually generated by synchronous generators with rotating electromagnetic fields.
The AC frequency of the generated output from a synchronous generator is defined generally by the relationship F=(P.times.N)/120 where:
F=AC frequency of generated output [cycles per second, or Hertz (HZ)];
P=number of magnetic poles; and
N=rotational velocity of magnetic field with respect to the stator armature windings (RPM).
In conventional modern synchronous generators, the magnetic field of the generator is fixed in position relative to the rotor of the generator. Thus, the frequency of the generated alternating current produced by the generator is determined by the rotational velocity of the power input shaft and the rotor of the generator.
A great deal of effort has been expended by the electrical power generating industry in the attempt to provide generated AC power at essentially constant AC frequencies by attempting to maintain the rotational speed of the rotor of synchronous generators at a precise fixed speed, with only partial success. Even the best rotational speed governors permit undesirable deviations from the ideal synchronous speed, resulting in generated electrical power with undesirably varying AC frequency.
Many attempts have been made to provide techniques to enable the AC frequency of the generated output from a synchronous generator to be independent of, or at least less dependent on, the rotational speed of the rotor of the generator. One approach has been to use complex electronic circuitry connected to the output of the generator. This electronic circuitry processes the full output of the generator to convert the varying or wild frequency AC output from the generator into a precisely fixed frequency AC output from the electronic inverter. While the approach may have been successful for some applications, the prohibitive cost of such an electronic converter restricts its use to only a limited number of critical applications.
Another approach has been to provide a technique for maintaining a constant rotational velocity of the magnetic field with respect to the stator armature windings of the generator, independently of the rotational speed of the power input shaft and rotor of the generator. In the latter approach, the rotor of the generator employs a polyphase AC electric winding similar to the electric winding found in the wound-rotor of a wound-rotor induction motor.
Applying AC power with the proper controllably variable AC frequency to the electric windings in the wound-rotor causes the electromagnetic field to rotate about the axis of the rotor at a controllably variable speed and direction to compensate for the variations in rotational speed of the rotor of the generator. Thus, the rotational velocity of the main electromagnetic field can be maintained essentially constant with respect to the stator armature windings of the generator, and the AC frequency of the generated output remains essentially constant
For a more detailed explanation of the latter approach, reference may be made to U.S. Pat. Nos. 2,659,044 and 4,246,531 which teach substantially similar arrangements. U.S. Pat. No. 2,659,044 discloses an apparatus for varying the frequency of the AC applied to the rotor windings in order to maintain the frequency of the generated AC output at somewhat constant value.
However, neither the apparatus in U.S. Pat. No. 2,659,044, nor in U.S. Pat. No. 4,246,531, can function at or near synchronous speed. In this regard, such apparatus can only function successfully when the rotors of both the main power generator and the field excitation power producing generator rotate at a speed substantially different from synchronous speed. Additionally, the apparatus in both patents require two generators to supplement the main power generator, and thus are relatively large and expensive.
U.S. Pat. No. 3,070,470 teaches an apparatus having an electronic cycloconverter mounted on the rotating shaft of the generator. The cycloconverter is electrically interposed between the source of the field excitation power and the wound-rotor electric windings comprising the main electromagnetic field windings of the main power generator.
The cycloconverter of U.S. Pat. No. 3,070,470 is controlled by electronic circuitry in such a manner to change the AC frequency of the output from the exciter generator to some controllably variable AC frequency. This controllably variable AC frequency is applied to the electric windings in the wound-rotor of the generator in such a manner as to compensate for the varying rotational speed of the generator rotor. The electronic apparatus disclosed in U.S. Pat. No. 3,070,470 requires multiple rotating pulse transformers and is unduly complex. Additionally, the apparatus does not provide means for successfully operating the generator in parallel with other sources of generated electrical power.
In U.S. Pat. Nos. 2,829,333, 4,510,433, and 3,084,324 there is disclosed various undesirably complex arrangements for determining the proper value of the variable AC frequency to be applied to the wound-rotor of a synchronous generator. In addition, they describe involved methods for generating the requisite compensating AC frequency, as well as complicated techniques for applying such AC frequency to the windings in the wound-rotor of the generator.
The apparatus disclosed in the preceding three patents include either rotor-mounted electronic circuits, or commutators and brushes, or other complex and awkward devices subject to wear and to failure in operation.
U.S. Pat. No. 4,400,659 teaches the use of a cycloconverter (described as a differential frequency-converter) mounted on the shaft of the generator, similar to U.S. Pat. No. 3,070,470. U.S. Pat. No. 4,400,659 teaches the use of a frequency detector to compare the generator output frequency with the AC frequency of the AC mains of electric utility power lines connected in parallel with the output of the generator.
The flaw in this arrangement is that once the generator output has been connected to the AC mains of the electric utility lines, the frequency detector senses only the AC frequency of the AC mains of the electric utility lines, and thus no comparison can be made.
U.S. Pat. No. 4,400,659 discloses an arrangement to measure the difference between the generated AC frequency output of the generator and the AC frequency of the parallel connected AC mains of the electric utility lines, and to make appropriate corrections when such generated AC frequency is different from that of the parallel connected AC mains of the electric utility lines. However, the flaw with this arrangement is that a synchronous generator connected in parallel with the AC mains of electric utility lines cannot generate an AC frequency different from that of the AC mains until after the generator has "pulled out" of synchronism with the AC mains. Once the generator has pulled out of synchronism with the AC mains, circuit protector means must immediately disconnect the synchronous generator from the AC mains before the synchronous generator windings are damaged by the undesirably large fault currents that will flow in the armature windings of the synchronous generator that is out of synchronism with connected AC mains. The magnitude of these currents is limited only by the small impedance of the generator stator armature windings opposing the full line voltage of the AC mains of the electric utility lines. In this regard, it can be seen that the apparatus taught in U.S. Pat. No. 4,400,659 does not provide means for successfully operating a variable speed, constant frequency (VSCF) generator in parallel with the AC mains of an electric utility power system.
One of the conditions that must be met before the output of a synchronous generator can be connected in parallel with electric utility power lines is that the output voltage of the incoming synchronous generator must be "in phase" with the voltage of the electric utility power lines.
Being "in phase" means that the difference between the instantaneous voltage of phase A of one system and phase A of the other system is zero. Similarly, at the same time, the instantaneous difference in voltage is zero between phases B and C of one system and the respective phases B and C of the other system. The differences between the instantaneous voltages of each corresponding pair of phases must be zero at the time the two three-phase systems are connected in parallel.
When conventional synchronous generators are to be connected to operate in parallel with electric utility lines or other synchronous generators, the speed, and thus the generated AC frequency, of the incoming generator is purposely made to be slightly different from that of the electric utility power line or other synchronous generator power system until one system catches up with the other system and the two systems are in phase. The circuit breaker connecting the two systems is closed at the instant when all three phases of the generator output are in phase with all three phases of the electric utility power lines or other synchronous generator power system.
In the case of variable speed, constant frequency (VSCF) generators, where the generated AC frequency of the VSCF generator is essentially constant, special steps must be taken to shift the phase of the output voltage of the VSCF generator to bring it in phase with the voltage of the AC mains of the electric utility lines or other synchronous generator systems before connection is made.
Once the output of the VSCF generator has been connected to the electric utility power lines, special steps must be taken to vary the "power angle" or "displacement angle" between the rotor magnetic field and the armature flux field of the variable speed constant frequency generator in order to control the power output of the synchronous generator while still maintaining a constant frequency output power.
The "power angle" or "displacement angle" is defined as the angle, in electrical degrees, between the angular orientation of the rotating armature magnetic flux vector and the angular orientation of the rotating field magnetic flux vector in the generator. The magnitude of the power angle or displacement angle determines the magnitude of the power output of the synchronous machine. In a synchronous motor, the field magnetic flux vector lags behind the armature magnetic flux vector. In a synchronous generator, the field magnetic flux vector leads the armature magnetic flux vector.
In the case of a conventional synchronous generator connected to the AC mains of an electrical utility power system, increasing the mechanical power input to the power input shaft of the generator causes the rotor and its associated magnetic field to pull ahead of the rotating synchronous armature magnetic flux vector, thereby increasing the power angle or displacement angle. This increase in the displacement angle between the angular orientation of the armature magnetic flux vector and the angular orientation of the rotor field magnetic flux results in increased power output from the generator.
The power angle or displacement angle increases until the power output from the generator, plus the internal power losses of the generator, equals the power input applied to the power input shaft of the generator. Reducing the mechanical power input to the generator results in a reduction of the power angle and a reduction in the power output from the generator. The AC frequency of the generator output does not vary during the momentary change of power angle or displacement angle. Only the relative angular positions of the rotor magnetic field vector and the armature flux field vector change.
A variable speed constant frequency (VSCF) generator operating in parallel with electric utility lines or other synchronous electric power systems requires special means, not heretofore disclosed in the prior art, to control the power angle, and thus the power output from the VSCF generator, under changing load conditions while still maintaining a constant frequency AC power output.
The internal distribution of power flow in a VSCF generator with AC excitation power in its magnetic field windings, while operating at speeds other than synchronous speed, is exactly like that of the well known wound-rotor induction motor frequency changer. The relative portion of the output power generated by the mechanical rotation of the mechanical structure of the rotor is defined as the ratio: (Rotor RPM]/Synchronous RPM].times.(Power Output). The remainder of the generated output power is supplied by the transformer action of the AC power in the magnetic field windings.
For example, if a 90 KW 4-pole 60 HZ (1800 RPM synchronous speed) VSCF generator is operating with an input shaft speed of 1200 RPM, [(1200/1800).times.90 KW=60 KW] of the output power is generated at 40 HZ in the armature windings by the rotation of the mechanical structure of the rotor, and [(600/1800).times.90 KW=30 KW] of the output power is generated at 20 HZ in the armature windings by the transformer action of the 20 HZ AC power in the field windings in the rotor of the generator, for a total of 90 KW at 60 HZ.
Therefore, a practical VSCF generator must provide means to generate the "makeup power" or the difference between the output power of the generator and the power generated directly by the mechanical rotation of the generator rotor. In the prior art, only the U.S. Pat. No. 3,070,470, without adequately solving the problem, addresses the distribution of power flow in a VSCF generator operating at speeds other than synchronous speed.
In summary, it is known in the prior art that the output AC frequency from a synchronous generator can be maintained at a precise fixed value, regardless of the variation of the rotational velocity of the input shaft and rotor structure of the generator, by applying controllably varying AC to the appropriately designed electromagnetic windings of the rotor of the generator. However, the techniques taught in the prior art are unduly large, complex, costly, or subject to unwanted early failures. Additionally, the prior art does not teach a practical method for controlling the phase relationship between the generated voltage of the VSCF generator and the voltage of other synchronous generators, nor does the prior art teach a practical method for controlling the power angle or displacement angle of a VSCF generator connected in parallel with other synchronous generators or the AC mains of electrical utility lines. Therefore, it would be highly desirable to have a new and improved variable speed constant frequency synchronous power generating system which can operate in parallel with other synchronous generators or with electric utility power lines for co-generation of power. Such a new and improved system should use a simple, practical VSCF generator which can operate successfully in either a stand-alone mode or in parallel with other synchronous generator systems.